this square you img to 1000px on a white background

square-image-gimp-script-fu.scm

; extract basename without ext
(define (drop-ext str)
	(unbreakupstr (butlast (strbreakup  str ".")) ".")	
)

(define (maxsize img)
	(max (car (gimp-image-width img)) (car (gimp-image-height img)))	
)

(define (offsets img)
	(let*
		(
			(width 		(car (gimp-image-width img)))
			(height 	(car (gimp-image-height img)))
			(maxSize 	(max  width height))
			(ofX (/ (- maxSize width) 2))
			(ofY (/ (- maxSize height) 2))
		)
		(list ofX ofY)			
	)
)


;=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
( define ( script-fu-square   sourceDirectory  )
   ( let*
      (
         ; Declare and Init local variables
         ( returnVal #f )
         ; Guess host OS based on directory path separator
         ( isLinux (  >  
            ( length ( strbreakup sourceDirectory "/" ) )
            ( length ( strbreakup sourceDirectory "\\" ) ) ) )
         ; Form path/file patternSource based on OS 
         ( patternSource ( if isLinux
            ( string-append sourceDirectory "/*.[jJ][pP][gG]" )
            ( string-append sourceDirectory "\\*.[jJ][pP][gG]" ) ) )
         ; List of files to be converted formatted for current Host
         ; O/S
         ( filelistSource ( cadr ( file-glob patternSource 1 ) ) )
         ; Place holders for image variables - updated per image
         ( theImage 0 )
				 ( theOrigLayer 0 )
				 ( theBgLayer 0 )
				 ( theSize 0 )
				 ( theOfs nil )
         ( theDrawable 0 )
         ( currentFile "" )
         ( baseName "" )

				
				 
      ) ; End declaration of Local Variables
      ;
      ; Run if images closed, message if not.
      ( if ( < 0 ( car ( gimp-image-list ) ) )
         ( gimp-message "Close open Images & Rerun" )
         ( begin
            ;
            ; Run within scope of let* and local variables
            ; 'baseName' is filename without .jpg extension
            ; 'outFilename' is filename with .xcf extension
            ; Step through each file in list with while loop.
            ( while ( not ( null? filelistSource ) )
               ( set! currentFile ( car filelistSource ) )
               ; Get open and get Image ID of current file
               ( set! theImage
                  ( car ( gimp-file-load RUN-NONINTERACTIVE 
                     currentFile currentFile ) ) )
		( begin
		;; --------------------------------------
		;; define the size
		(set! theSize (maxsize theImage))
		(set! theOfs (offsets theImage))
		;; add white bg layer
		(set! theBgLayer (car (gimp-layer-new theImage theSize theSize 1 "bg" 100 0)))
		(gimp-image-insert-layer theImage theBgLayer 0 1)
		(gimp-edit-fill theBgLayer 1)
		(set! theOrigLayer (vector-ref (cadr (gimp-image-get-layers theImage)) 0))
		(gimp-image-resize-to-layers theImage)	
		(gimp-layer-translate theOrigLayer (car theOfs) (cadr theOfs)) 
		(gimp-image-scale theImage 1000 1000)

		; Get / set Drawable ID, need it for file save.
		( set! theDrawable ( car 
		( gimp-image-merge-visible-layers theImage 0 ) ) )
		; Save file 
		( gimp-file-save 
		RUN-NONINTERACTIVE theImage theDrawable
		currentFile currentFile )
		;; --------------------------------------
		) ; End begin
             
							 
               ( gimp-image-delete theImage )
               ; Update while loop iteration parameter
               (set! filelistSource ( cdr filelistSource ) )
            ) ; End while
         ); End outer begin
      ) ; End outer if
      ( set! returnVal #t )
   ) ; End let*
) ; End define
;=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
( script-fu-register "script-fu-square" ; Function Name
   "1 ) Square JPG (Directory)"    ; Menu Label
   "This script is an interactive script to square all of the jpegs 
   in a source directory.  The script is designed to be run WITHOUT ANY IMAGE 
   LOADED. Runs from GIMP shell in Linux and Windows."
   "Yawo KPOTUFE"       ; Author
   "2016, Yawo KPOTUFE" ; Copyright
   "November 2016"          ; Creation Date
   ""                   ; Valid Image Type - No Image required
    ; We actually don't want any images open when we run this
    ;   script, so it must be available from the menu when an
    ;   image is not loaded.  This script will determine the IDs
    ;   of the Image and Drawable itself rather than having them
    ;   passed as parameters.
   ; Interactive widgets
   SF-DIRNAME "JPG Originals (source) Directory" ""
) ; End script-fu-register
( script-fu-menu-register 
   "script-fu-square" "<Image>/Square")
;=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=